Used in an index, unless the column is a varchar, nvarchar, or varbinary data type, the data type is not changed, the new size is equal to or larger than the old size, and the index is not the result of a PRIMARY KEY constraint.

Used in statistics generated by the CREATE STATISTICS statement unless the column is a varchar, nvarchar, or varbinary data type, the data type is not changed, and the new size is equal to or greater than the old size, or if the column is changed from not null to null. First, remove the statistics using the DROP STATISTICS statement. Statistics that are automatically generated by the query optimizer are automatically dropped by ALTER COLUMN.

Used in a PRIMARY KEY or [FOREIGN KEY] REFERENCES constraint.

Used in a CHECK or UNIQUE constraint. However, changing the length of a variable-length column used in a CHECK or UNIQUE constraint is allowed.

Associated with a default definition. However, the length, precision, or scale of a column can be changed if the data type is not changed.

The data type of text, ntext and image columns can be changed only in the following ways:

text to varchar(max), nvarchar(max), or xml

ntext to varchar(max), nvarchar(max), or xml

image to varbinary(max)

Some data type changes may cause a change in the data. For example, changing an nchar or nvarchar column to char or varchar may cause the conversion of extended characters. For more information, see CAST and CONVERT (Transact-SQL). Reducing the precision or scale of a column may cause data truncation.

The data type of a column of a partitioned table cannot be changed.

column_name

Is the name of the column to be altered, added, or dropped. column_name can be a maximum of 128 characters. For new columns, column_name can be omitted for columns created with a timestamp data type. The name timestamp is used if no column_name is specified for a timestamp data type column.

[ type_schema_name. ] type_name

Is the new data type for the altered column, or the data type for the added column. type_name cannot be specified for existing columns of partitioned tables. type_name can be any one of the following:

A SQL Server system data type.

An alias data type based on a SQL Server system data type. Alias data types are created with the CREATE TYPE statement before they can be used in a table definition.

A .NET Framework user-defined type, and the schema to which it belongs. .NET Framework user-defined types are created with the CREATE TYPE statement before they can be used in a table definition.

The following are criteria for type_name of an altered column:

The previous data type must be implicitly convertible to the new data type.

type_name cannot be timestamp.

ANSI_NULL defaults are always on for ALTER COLUMN; if not specified, the column is nullable.

ANSI_PADDING padding is always ON for ALTER COLUMN.

If the modified column is an identity column, new_data_type must be a data type that supports the identity property.

The current setting for SET ARITHABORT is ignored. ALTER TABLE operates as if ARITHABORT is set to ON.

Note

If the COLLATE clause is not specified, changing the data type of a column will cause a collation change to the default collation of the database.

Applies only to the varchar, nvarchar, and varbinary data types for storing 2^31-1 bytes of character, binary data, and of Unicode data.

xml_schema_collection

Applies only to the xml data type for associating an XML schema with the type. Before typing an xml column to a schema collection, the schema collection must first be created in the database by using CREATE XML SCHEMA COLLECTION.

The COLLATE clause can be used to change the collations only of columns of the char, varchar, nchar, and nvarchar data types. To change the collation of a user-defined alias data type column, you must execute separate ALTER TABLE statements to change the column to a SQL Server system data type and change its collation, and then change the column back to an alias data type.

ALTER COLUMN cannot have a collation change if one or more of the following conditions exist:

Specifies whether the column is a sparse column or can accept null values. If the column to be altered is a sparse column, you must explicitly specify this property or the column will revert to a nonsparse column. Sparse columns cannot be designated as NOT NULL. Converting a column from sparse to nonsparse or from nonsparse to sparse locks the table for the duration of the command execution.

For additional restrictions and more information about sparse columns and nullability, see Using Sparse Columns.

Columns that do not allow null values can be added with ALTER TABLE only if they have a default specified or if the table is empty. NOT NULL can be specified for computed columns only if PERSISTED is also specified. If the new column allows null values and no default is specified, the new column contains a null value for each row in the table. If the new column allows null values and a default definition is added with the new column, WITH VALUES can be used to store the default value in the new column for each existing row in the table.

If the new column does not allow null values and the table is not empty, a DEFAULT definition must be added with the new column, and the new column automatically loads with the default value in the new columns in each existing row.

NULL can be specified in ALTER COLUMN to force a NOT NULL column to allow null values, except for columns in PRIMARY KEY constraints. NOT NULL can be specified in ALTER COLUMN only if the column contains no null values. The null values must be updated to some value before the ALTER COLUMN NOT NULL is allowed, for example:

When you create or alter a table with the CREATE TABLE or ALTER TABLE statements, the database and session settings influence and possibly override the nullability of the data type that is used in a column definition. We recommend that you always explicitly define a column as NULL or NOT NULL for noncomputed columns.

If you add a column with a user-defined data type, we recommend that you define the column with the same nullability as the user-defined data type and specify a default value for the column. For more information, see CREATE TABLE (Transact-SQL).

Note

If NULL or NOT NULL is specified with ALTER COLUMN, new_data_type [(precision [, scale ])] must also be specified. If the data type, precision, and scale are not changed, specify the current column values.

[ {ADD | DROP} ROWGUIDCOL ]

Specifies the ROWGUIDCOL property is added to or dropped from the specified column. ROWGUIDCOL indicates that the column is a row GUID column. Only one uniqueidentifier column per table can be designated as the ROWGUIDCOL column, and the ROWGUIDCOL property can be assigned only to a uniqueidentifier column. ROWGUIDCOL cannot be assigned to a column of a user-defined data type.

ROWGUIDCOL does not enforce uniqueness of the values that are stored in the column and does not automatically generate values for new rows that are inserted into the table. To generate unique values for each column, either use the NEWID function on INSERT statements or specify the NEWID function as the default for the column.

[ {ADD | DROP} PERSISTED ]

Specifies that the PERSISTED property is added to or dropped from the specified column. The column must be a computed column that is defined with a deterministic expression. For columns specified as PERSISTED, the Database Engine physically stores the computed values in the table and updates the values when any other columns on which the computed column depends are updated. By marking a computed column as PERSISTED, you can create indexes on computed columns defined on expressions that are deterministic, but not precise. For more information, see Creating Indexes on Computed Columns.

Any computed column that is used as a partitioning column of a partitioned table must be explicitly marked PERSISTED.

Specifies that the column to be added or dropped is a sparse column. The storage of sparse columns is optimized for null values. Sparse columns cannot be designated as NOT NULL. Converting a column from sparse to nonsparse or from nonsparse to sparse locks the table for the duration of the command execution. You may need to use the REBUILD clause to reclaim any space savings.

Important

You must specify the SPARSE property each time you alter the column, or the column will revert to a nonsparse column.

Specifies whether the data in the table is or is not validated against a newly added or re-enabled FOREIGN KEY or CHECK constraint. If not specified, WITH CHECK is assumed for new constraints, and WITH NOCHECK is assumed for re-enabled constraints.

If you do not want to verify new CHECK or FOREIGN KEY constraints against existing data, use WITH NOCHECK. We do not recommend doing this, except in rare cases. The new constraint will be evaluated in all later data updates. Any constraint violations that are suppressed by WITH NOCHECK when the constraint is added may cause future updates to fail if they update rows with data that does not comply with the constraint.

The query optimizer does not consider constraints that are defined WITH NOCHECK. Such constraints are ignored until they are re-enabled by using ALTER TABLE <table> WITH CHECK CHECK CONSTRAINT ALL.

ADD

Specifies that one or more column definitions, computed column definitions, or table constraints are added.

DROP { [ CONSTRAINT ] constraint_name | COLUMN column_name }

Specifies that constraint_name or column_name is removed from the table. Multiple columns and constraints can be listed.

The user-defined or system-supplied name of the constraint can be determined by querying the sys.check_constraint, sys.default_constraints, sys.key_constraints, and sys.foreign_keys catalog views.

A PRIMARY KEY constraint cannot be dropped if an XML index exists on the table.

A column cannot be dropped when it is:

Used in an index.

Used in a CHECK, FOREIGN KEY, UNIQUE, or PRIMARY KEY constraint.

Associated with a default that is defined with the DEFAULT keyword, or bound to a default object.

Bound to a rule.

Note

Dropping a column does not reclaim the disk space of the column. You may have to reclaim the disk space of a dropped column when the row size of a table is near, or has exceeded, its limit. Reclaim space by creating a clustered index on the table or rebuilding an existing clustered index by using ALTER INDEX.

WITH <drop_clustered_constraint_option>

Specifies that one or more drop clustered constraint options are set.

MAXDOP = max_degree_of_parallelism

Overrides the max degree of parallelism configuration option only for the duration of the operation. For more information, see max degree of parallelism Option.

Use the MAXDOP option to limit the number of processors used in parallel plan execution. The maximum is 64 processors.

max_degree_of_parallelism can be one of the following values:

1

Suppresses parallel plan generation.

>1

Restricts the maximum number of processors used in a parallel index operation to the specified number.

0 (default)

Uses the actual number of processors or fewer based on the current system workload.

Parallel index operations are available only in SQL Server Enterprise, Developer, and Evaluation editions.

ONLINE = { ON | OFF }

Specifies whether underlying tables and associated indexes are available for queries and data modification during the index operation. The default is OFF. REBUILD can be performed as an ONLINE operation.

ON

Long-term table locks are not held for the duration of the index operation. During the main phase of the index operation, only an Intent Share (IS) lock is held on the source table. This enables queries or updates to the underlying table and indexes to continue. At the start of the operation, a Shared (S) lock is held on the source object for a very short time. At the end of the operation, for a short time, an S (Shared) lock is acquired on the source if a nonclustered index is being created; or an SCH-M (Schema Modification) lock is acquired when a clustered index is created or dropped online and when a clustered or nonclustered index is being rebuilt. ONLINE cannot be set to ON when an index is being created on a local temporary table. Only single-threaded HEAP rebuild operation is allowed.

OFF

Table locks are applied for the duration of the index operation. An offline index operation that creates, rebuilds, or drops a clustered index, or rebuilds or drops a nonclustered index, acquires a Schema modification (Sch-M) lock on the table. This prevents all user access to the underlying table for the duration of the operation. An offline index operation that creates a nonclustered index acquires a Shared (S) lock on the table. This prevents updates to the underlying table but allows read operations, such as SELECT statements. Allows multi-threaded HEAP rebuild operations.

Specifies a location to move the data rows currently in the leaf level of the clustered index. The table is moved to the new location. This option applies only to constraints that create a clustered index.

Note

In this context, default is not a keyword. It is an identifier for the default filegroup and must be delimited, as in MOVE TO "default" or MOVE TO [default]. If "default" is specified, the QUOTED_IDENTIFIER option must be ON for the current session. This is the default setting. For more information, see SET QUOTED_IDENTIFIER (Transact-SQL).

{ CHECK | NOCHECK } CONSTRAINT

Specifies that constraint_name is enabled or disabled. This option can only be used with FOREIGN KEY and CHECK constraints. When NOCHECK is specified, the constraint is disabled and future inserts or updates to the column are not validated against the constraint conditions. DEFAULT, PRIMARY KEY, and UNIQUE constraints cannot be disabled.

ALL

Specifies that all constraints are either disabled with the NOCHECK option or enabled with the CHECK option.

{ ENABLE | DISABLE } TRIGGER

Specifies that trigger_name is enabled or disabled. When a trigger is disabled it is still defined for the table; however, when INSERT, UPDATE, or DELETE statements are executed against the table, the actions in the trigger are not performed until the trigger is re-enabled.

ALL

Specifies that all triggers in the table are enabled or disabled.

trigger_name

Specifies the name of the trigger to disable or enable.

{ ENABLE | DISABLE } CHANGE_TRACKING

Specifies whether change tracking is enabled disabled for the table. By default, change tracking is disabled.

Reassigns all data of a table as a partition to an already-existing partitioned table.

Switches a partition from one partitioned table to another.

Reassigns all data in one partition of a partitioned table to an existing non-partitioned table.

If table is a partitioned table, source_partition_number_expression must be specified. If target_table is partitioned, target_partition_number_expression must be specified. If reassigning a table's data as a partition to an already-existing partitioned table, or switching a partition from one partitioned table to another, the target partition must exist and it must be empty.

If reassigning one partition's data to form a single table, the target table must already be created and it must be empty. Both the source table or partition, and the target table or partition, must reside in the same filegroup. The corresponding indexes, or index partitions, must also reside in the same filegroup. Many additional restrictions apply to switching partitions. For more information, see Transferring Data Efficiently by Using Partition Switching. table and target_table cannot be the same. target_table can be a multi-part identifier.

source_partition_number_expression and target_partition_number_expression are constant expressions that can reference variables and functions. These include user-defined type variables and user-defined functions. They cannot reference Transact-SQL expressions.

ALTER TABLE with the SET FILESTREAM_ON clause will succeed only if the table has no FILESTREAM columns. The FILESTREAM columns can be added by using a second ALTER TABLE statement.

If partition_scheme_name is specified, the rules for CREATE TABLE apply. The table should already be partitioned for row data, and its partition scheme must use the same partition function and columns as the FILESTREAM partition scheme.

filestream_filegroup_name specifies the name of a FILESTREAM filegroup. The filegroup must have one file that is defined for the filegroup by using a CREATE DATABASE or ALTER DATABASE statement, or an error is raised.

"default" specifies the FILESTREAM filegroup with the DEFAULT property set. If there is no FILESTREAM filegroup, an error is raised.

"NULL" specifies that all references to FILESTREAM filegroups for the table will be removed. All FILESTREAM columns must be dropped first. You must use SET FILESTREAM_ON="NULL" to delete all FILESTREAM data that is associated with a table.

SET ( LOCK_ESCALATION = { AUTO | TABLE | DISABLE } )

Specifies the allowed methods of lock escalation for a table.

AUTO

This option allows SQL Server Database Engine to select the lock escalation granularity that is appropriate for the table schema.

If the table is partitioned, lock escalation will be allowed to partition. After the lock is escalated to the partition level, the lock will not be escalated later to TABLE granularity.

If the table is not partitioned, the lock escalation will be done to the TABLE granularity.

TABLE

Lock escalation will be done at table-level granularity regardless whether the table is partitioned or not partitioned. This behavior is the same as in SQL Server 2005. TABLE is the default value.

DISABLE

Prevents lock escalation in most cases. Table-level locks are not completely disallowed. For example, when you are scanning a table that has no clustered index under the serializable isolation level, Database Engine must take a table lock to protect data integrity.

REBUILD

Use the REBUILD WITH syntax to rebuild an entire table including all the partitions in a partitioned table. If the table has a clustered index, the REBUILD option rebuilds the clustered index. REBUILD can be performed as an ONLINE operation.

Use the REBUILD PARTITION syntax to rebuild a single partition in a partitioned table.

PARTITION = ALL

Rebuilds all partitions when changing the partition compression settings.

REBUILD WITH ( <rebuild_option> )

All options apply to a table with a clustered index. If the table does not have a clustered index, the heap structure is only affected by some of the options.

When a specific compression setting is not specified with the REBUILD operation, the current compression setting for the partition is used. To return the current setting, query the data_compression column in the sys.partitions catalog view.

Is the name of the column set. A column set is an untyped XML representation that combines all of the sparse columns of a table into a structured output. A column set cannot be added to a table that contains sparse columns. For more information about column sets, see Using Column Sets.

If there are any execution plans in the procedure cache that reference the table, ALTER TABLE marks them to be recompiled on their next execution.

Changing the Size of a Column

You can change the length, precision, or scale of a column by specifying a new size for the column data type in the ALTER COLUMN clause. If data exists in the column, the new size cannot be smaller than the maximum size of the data. Also, the column cannot be defined in an index, unless the column is a varchar, nvarchar, or varbinary data type and the index is not the result of a PRIMARY KEY constraint. See example P.

Locks and ALTER TABLE

The changes specified in ALTER TABLE are implemented immediately. If the changes require modifications of the rows in the table, ALTER TABLE updates the rows. ALTER TABLE acquires a schema modify lock on the table to make sure that no other connections reference even the metadata for the table during the change, except online index operations that require a very short SCH-M lock at the end. In an ALTER TABLE…SWITCH operation, the lock is acquired on both the source and target tables. The modifications made to the table are logged and fully recoverable. Changes that affect all the rows in very large tables, such as dropping a column or adding a NOT NULL column with a default, can take a long time to complete and generate many log records. These ALTER TABLE statements should be executed with the same care as any INSERT, UPDATE, or DELETE statement that affects many rows.

Parallel Plan Execution

In SQL Server 2008 Enterprise, the number of processors employed to run a single ALTER TABLE ADD (index based) CONSTRAINT or DROP (clustered index) CONSTRAINT statement is determined by the max degree of parallelism configuration option and the current workload. If the Database Engine detects that the system is busy, the degree of parallelism of the operation is automatically reduced before statement execution starts. You can manually configure the number of processors that are used to run the statement by specifying the MAXDOP option.

Partitioned Tables

In addition to performing SWITCH operations that involve partitioned tables, ALTER TABLE can be used to change the state of the columns, constraints, and triggers of a partitioned table just like it is used for nonpartitioned tables. However, this statement cannot be used to change the way the table itself is partitioned. To repartition a partitioned table, use ALTER PARTITION SCHEME and ALTER PARTITION FUNCTION. Additionally, you cannot change the data type of a column of a partitioned table.

Restrictions on Tables with Schema-Bound Views

The restrictions that apply to ALTER TABLE statements on tables with schema-bound views are the same as the restrictions currently applied when modifying tables with a simple index. Adding a column is allowed. However, removing or changing a column that participates in any schema-bound view is not allowed. If the ALTER TABLE statement requires changing a column used in a schema-bound view, ALTER TABLE fails and the Database Engine raises an error message. For more information about schema binding and indexed views, see CREATE VIEW (Transact-SQL).

Adding or removing triggers on base tables is not affected by creating a schema-bound view that references the tables.

Indexes and ALTER TABLE

Indexes created as part of a constraint are dropped when the constraint is dropped. Indexes that were created with CREATE INDEX must be dropped with DROP INDEX. The ALTER INDEX statement can be used to rebuild an index part of a constraint definition; the constraint does not have to be dropped and added again with ALTER TABLE.

All indexes and constraints based on a column must be removed before the column can be removed.

When a constraint that created a clustered index is deleted, the data rows that were stored in the leaf level of the clustered index are stored in a nonclustered table. You can drop the clustered index and move the resulting table to another filegroup or partition scheme in a single transaction by specifying the MOVE TO option. The MOVE TO option has the following restrictions:

MOVE TO is not valid for indexed views or nonclustered indexes.

The partition scheme or filegroup must already exist.

If MOVE TO is not specified, the table will be located in the same partition scheme or filegroup as was defined for the clustered index.

When you drop a clustered index, you can specify ONLINE = ON option so the DROP INDEX transaction does not block queries and modifications to the underlying data and associated nonclustered indexes.

ONLINE = ON has the following restrictions:

ONLINE = ON is not valid for clustered indexes that are also disabled. Disabled indexes must be dropped by using ONLINE = OFF.

Only one index at a time can be dropped.

ONLINE = ON is not valid for indexed views, nonclustered indexes or indexes on local temp tables.

Temporary disk space equal to the size of the existing clustered index is required to drop a clustered index. This additional space is released as soon as the operation is completed.

Note

The options listed under <drop_clustered_constraint_option> apply to clustered indexes on tables and cannot be applied to clustered indexes on views or nonclustered indexes.

Replicating Schema Changes

By default, when you run ALTER TABLE on a published table at a SQL Server Publisher, that change is propagated to all SQL Server Subscribers. This functionality has some restrictions and can be disabled. For more information, see Making Schema Changes on Publication Databases.

Data Compression

System tables cannot be enabled for compression. If the table is a heap, the rebuild operation for ONLINE mode will be single threaded. Use OFFLINE mode for a multi-threaded heap rebuild operation. For a more information about data compression, see Creating Compressed Tables and Indexes.

ALTER TABLE permissions apply to both tables involved in an ALTER TABLE SWITCH statement. Any data that is switched inherits the security of the target table.

If any columns in the ALTER TABLE statement are defined to be of a common language runtime (CLR) user-defined type or alias data type, REFERENCES permission on the type is required.

Adding a column that updates the rows of the table requires UPDATE permission on the table. For example, adding a NOT NULL column with a default value or adding an identity column when the table is not empty.

E. Adding an unverified CHECK constraint to an existing column

The following example adds a constraint to an existing column in the table. The column has a value that violates the constraint. Therefore, WITH NOCHECK is used to prevent the constraint from being validated against existing rows, and to allow for the constraint to be added.

F. Adding a DEFAULT constraint to an existing column

The following example creates a table with two columns and inserts a value into the first column, and the other column remains NULL. A DEFAULT constraint is then added to the second column. To verify that the default is applied, another value is inserted into the first column, and the table is queried.

G. Adding several columns with constraints

The following example adds several columns with constraints defined with the new column. The first new column has an IDENTITY property. Each row in the table has new incremental values in the identity column.

H. Adding a nullable column with default values

The following example adds a nullable column with a DEFAULT definition, and uses WITH VALUES to provide values for each existing row in the table. If WITH VALUES is not used, each row has the value NULL in the new column.

I. Disabling and re-enabling a constraint

The following example disables a constraint that limits the salaries accepted in the data. NOCHECK CONSTRAINT is used with ALTER TABLE to disable the constraint and allow for an insert that would typically violate the constraint. CHECK CONSTRAINT re-enables the constraint.

K. Switching partitions between tables

The following example creates a partitioned table, assuming that partition scheme myRangePS1 is already created in the database. Next, a non-partitioned table is created with the same structure as the partitioned table and on the same filegroup as PARTITION 2 of table PartitionTable. The data of PARTITION 2 of table PartitionTable is then switched into table NonPartitionTable.

L. Disabling and re-enabling a trigger

The following example uses the DISABLE TRIGGER option of ALTER TABLE to disable the trigger and allow for an insert that would typically violate the trigger. ENABLE TRIGGER is then used to re-enable the trigger.

M. Creating a PRIMARY KEY constraint with index options

The following example creates the PRIMARY KEY constraint PK_TransactionHistoryArchive_TransactionID and sets the options FILLFACTOR, ONLINE, and PAD_INDEX. The resulting clustered index will have the same name as the constraint.

O. Adding and dropping a FOREIGN KEY constraint

The following example creates the table ContactBackup, and then alters the table, first by adding a FOREIGN KEY constraint that references the table Person, then by dropping the FOREIGN KEY constraint.

P. Changing the size of a column

The following example increases the size of a varchar column and the precision and scale of a decimal column. Because the columns contain data, the column size can only be increased. Also notice that col_a is defined in a unique index. The size of col_a can still be increased because the data type is a varchar and the index is not the result of a PRIMARY KEY constraint.